It isn't caused the graphics engine really, it is the technique used to overlay the very detailed 3D cockpit over the normal 3D model in the world. The cockpit model is only ever drawn in the F1 view as it is very Polygon intensive.
One fix would be to merge the Cockpit model into the main 3d model - However the polygons used for each aircraft then would likely cripple most computers. A better fix could possibly be for the Cockpit model to react to outside light sources too, the line might not appear then?
Either way there are no easy fixes due to the current architecture.
Nate